For thousands of years, two intelligent hominid species shared the European continent: early humans and Neanderthals. About 30,000 years ago, Neanderthals went extinct. Now one study suggests a possible reason: The Neanderthal population was very small, and very interrelated.

In the new study published today in Science, a group of European researchers sequenced five mitochondrial DNA taken from Neanderthal bones, some of which were 70,000 years old. They used a special technique to extract Neanderthal DNA from other kinds of materials that get mixed into fossils over time. What they discovered was that this DNA, taken from various regions around Europe, had many features in common.

According to Science:

[The researchers Jeffrey Good and Adrian Briggs] found 55 places out of the 16,565 bases where the mitochondrial genomes varied across the six ancient samples. On average, they found 20 differences between any two samples. In modern humans, about 60 differences exist between any two samples, making Neandertals about one-third as diverse.

Based on this lack of variation, the researchers were able to extrapolate roughly how large the population of Neanderthals might have been between 70,000 and 38,000 years ago. It appears that there were probably only about 7,000 Neandertals in Europe when homo sapiens arrived. The researchers speculate that this low population number may also have contributed to this species' eventual extinction.

The number also helps to explain other archaeological evidence, or rather the lack thereof. Scientists have found very few remains of Neanderthal cultures, and very few fossils of them as well. What this new research suggests, however, is that this tiny band spread very far across Europe. There may not have been a lot of them, they were good travelers.

Some scientists speculate that it might not be appropriate to categorize Neanderthals as a separate species at all. One might view them as an extreme variation on homo sapiens, which evolved squat bodies and thick brows to cope with the extreme cold of the European winter. Homo sapiens, which evolved in Africa, would have looked dramatically different from their pale, squat Neanderthal cousins. For now, however, Neanderthals are classified as their own species. And now it looks as if they were always a very tiny, marginal group.